Exemplary embodiments of the present invention relate generally to weighing systems and metal detection systems. More particularly, an exemplary embodiment of the present invention is directed to a combined weighing/metal-detection system and method, wherein a checkweighing scale is combined with a metal detector.
Checkweighing scales are a key element for quality assurance in the production of pharmaceutical or cosmetic products, food products, and beverages, as well as in the logistics, the chemical, auto parts, and metalworking industries. They not only improve the utilization of available resources, but also help in meeting national regulations, Weights & Measures requirements, and industry standards. An effective checkweighing system gives protection against product failures and lowers the overall operating costs.
A dynamic checkweigher is a system which determines the weight of articles while they are transported over a scale within a production line, which classifies the articles in accordance with predefined weight classes, and subsequently sorts or diverts the articles according to their respective classifications. Checkweighers are used in many diverse fields of application, including for example:                checking articles for under or overweight;        checking packaged goods for compliance with legal requirements regarding net weight of contents;        reduction of product waste by using the weight values obtained from the checkweigher for the adjustment of the filling machines;        classification of products according to weight;        measuring and recording the performance of the production facility or production line; and        parts count verification based on weight.        
When checkweighers are used, 100% of the articles of a product line are weighed. In the process, all of the production data are collected for product counts, traceability of production lots, or production statistics.
A checkweigher normally consists of an infeed belt, weighing belt, discharge belt with sorting device, and a weighing terminal with a user interface. The weighing belt, which is arranged between infeed and discharge belt, rests on a weighing cell, which weighs the product as it travels over the weighing belt. The two most frequently used types of weighing cells for checkweighers are strain gauge load cells or weighing cells based on the principle of electromagnetic force compensation.
Like checkweighers, metal detectors are also among the key components for effective quality assurance. Metal detectors are employed by industrial users for the detection of contaminants in products, such as for example lead shot in meet, wire snips in grain, metal fragments from repairs on the production line, or contaminants picked up in the production process.
An industrial metal-detection system is a highly developed instrument serving to detect and sort out foreign bodies. The detection capability includes ferrous metals including high-grade steel, and non-ferrous metals such as brass, copper, aluminum and lead. A typical metal-detection system consists of four main components: sensor head, transport system, operating unit, and an automatic diverter system.
Sensor heads of the current state of the art fall primarily into two categories based on different detection technologies. A first category of sensor heads is equipped with symmetrical coils. Three coils are wound exactly parallel to each other on a non-metallic carrier body. The coil in the middle carries an alternating current of high frequency, thus generating an alternating magnetic field. The coils on either side act as receivers and, as a result of the symmetric arrangement, as long as no metallic contaminants pass through the detector, identical voltages are induced in the coils. If a product containing metallic contaminants passes through the coil arrangement, the high-frequency field is disturbed first in one receiver coil and then in the other. This causes transient changes of the induced voltage in the receiver coils, and the resultant signal can be processed to register the detection of the metallic contaminant.
The second category of sensor heads is used for the inspection of products in packages containing aluminum foil. The product that is to be inspected is exposed to a strong magnetic field whereby the metallic contaminant is magnetized. This magnetization is detected by way of a small voltage that is induced in a receiver coil. Sensor heads in this category have a significantly higher detection sensitivity for magnetic substances than for non-magnetic substances.
To protect the metal detector from being disturbed by interference due to metallic components or machines in the vicinity, the sensor head is enclosed and shielded in a metallic housing, normally of aluminum. The metallic housing also serves to enhance strength and rigidity and thus contributes significantly to the overall performance of the metal detector.
In spite of the shielding with a metallic housing, a part of the high-frequency magnetic field can escape from the opening of the metal detector to the outside and compromise the function of the metal detector, if the magnetic field is disturbed by metallic objects. To achieve optimal results with the metal detector, no metallic objects may be present within a certain range of the metal detector opening. This range is referred to as metal-free zone (MFZ). For a reliable product inspection this factor needs to be taken into account.
A metal detector of this kind and solutions to the aforementioned problems are described in EP 0 536 288.
For the in-motion weighing process, the weighing objects are preferably arranged in a uniform, repeatable manner, spaced apart at suitable regular intervals and in correct alignment. Thus, the checkweighing scale lends itself as the optimal platform in which to incorporate further inspection devices such as, for example, metal detectors.
Combined systems are more convenient to install and to operate. Furthermore, a combined system is in general more cost-effective than two systems that are purchased separately and then installed together in the production line. With a common entry of the article parameters on one operator panel for both parts of the combined system, the risk of operator errors can be reduced and change-overs between articles can be performed faster. In addition, costs for operator training, maintenance and cleaning are reduced.
At the current state of the art, checkweighing scales and metal-detection devices are arranged in separation from each other, following each other sequentially in a transport direction. For example in US 2007/0207242, a quality control system is described which consists of a detection unit for foreign objects and a weighing system. Both of these units of the quality control system have their own separate belt conveyors and are arranged in separation from each other in the quality control system. One operator unit serves to control both units of the system, which represents a big advantage to the user. Also, there is only one sorting device in the system, which removes products that are not meeting the prescribed quality assurance criteria of the system. These criteria can include weight tolerances or metallic contaminants. Other systems with separate units are described in JP 2009/109346 and JP 11 183 240. A big disadvantage of a separate arrangement is the long transport path through both units and the large amount of space required for it at the place of installation.
A system which likewise includes a metal detector, a weighing cell and a belt conveyor is disclosed in JP 4 052 521. The metal detector and the belt conveyor are mounted on the same chassis frame and are supported together by the weighing cell which, in turn, rests on a support frame on the floor.
According to EP 0536 288, a metal detector is equipped with a field-generating coil consisting in most cases of copper which, together with the metal housing, puts a lot of weight on the weighing cell. This so-called pre-load has a negative effect on the accuracy of weighing results, and the weighing cell has to be designed for a capacity that includes the weight of the metal detector. The pre-load, also referred to as tare, is that part of the weighing load which is not the object of the weight measurement, but which cannot be separated from the actual weight that is of interest. The consequence of this is that only articles above a certain minimum weight can be weight-checked.
An exemplary embodiment of the present invention may have the advantage of minimizing the amount of space required for a combined weighing/metal-detection system. An additional advantage may be to provide optimal measuring conditions for the weighing cells of the check-weighing scale, so as to optimize on the one hand the accuracy of the weight measurements and on the other hand the design of the weighing cell.
An exemplary embodiment of the invention may provide a combined weighing/metal-detection system which comprises at least one support frame through which the weighing/metal-detection system is supported on the floor, further at least two weighing cells which provide the weight value and which are arranged at mounting locations on the at least one support frame, and at least one conveyor device which, in the operating mode of the weighing/metal-detection system, rests on the weighing cells. The weighing/metal-detection system further comprises a metal detector with a passage opening, wherein the conveyor device is arranged to pass through the opening of the metal detector. Relative to the transport direction of the conveyor, the at least two weighing cells are arranged, respectively, on opposite sides of the metal detector, preferably close to the upstream and downstream ends of the conveyor device. The metal detector is supported on the at least one support frame at mounting locations different from those of the weighing cells, so that the weighing cells are not carrying the pre-load of the metal detector and the two functions of weighing and of detecting metallic contaminants can occur on the same conveyor device close together in time.
By supporting the metal detector directly on a support frame, separately from the support of the conveyor device, the pre-load on the weighing cells is significantly reduced. In contrast to the solutions of the known art, the weight of the metal detector no longer contributes to the pre-load. The weight of the metal detector, which previously rested on the weighing cells as a constant base load, is no longer of concern, so that special design measures to achieve a light-weight metal detector have become unnecessary. Due to the lower pre-load, the weighing cells used in systems of the known art can be replaced by at least two weighing cells that are matched to the new pre-load conditions. The resolution, i.e., the ability to discriminate between two closely adjacent measurement values, is thereby increased.
Due to the fact that the conveyor device is supported separately from the metal detector, by way of weighing cells on a support frame, vibrations caused by the operation of the conveyor device are damped in their propagation to the support frame.
The detection performance of the metal detector is improved in comparison to those known art designs where the conveyor device and the metal detector together rest on the load cells which are mounted on the support frame.
Since the conveyor device on which the articles are weighed extends through the metal detector, the operations of weighing and of detecting metallic contaminants can occur simultaneously during the passage of the weighing object over the conveyor device. The overall length of a combined weighing/metal-detection system is thus significantly shortened.
According to a further developed embodiment of the invention, the metal detector and the conveyor device are supported through separate mounting locations on one and the same support frame. By using a common support frame, the amount of space required is minimized and when the system is installed at the operating location, the alignment and the attachment to the floor are simplified.
In an advantageous further developed embodiment of the invention, the support frame has dedicated mounting locations for the attachment of a metal detector and at least two dedicated mounting locations for the attachment of weighing cells which, in turn, are arranged so that they can receive the conveyor device. The weighing system and the metal-detection system are supported separately through their respective mounting locations on the same support frame. The metal detector is centrally mounted midway on the support frame, and the conveyor device is mounted on at least two weighing cells, which are preferably arranged at the opposite outer ends of the support frame. The separate support arrangement allows the pre-load on the weighing cells to be reduced by the weight of the metal detector. The arrangement and use of two or more weighing cells further offers cost savings, as fewer weighing cells need to be installed, for example, with narrow conveyor belts or if the articles are always positioned in the middle of the conveyor belt, in other words, in any applications where the weighing cells are not subject to the problem of eccentric loading.
In a further embodiment of the invention, the metal detector is supported through mounting locations on a first support frame, and the conveyor device is supported through at least two further mounting locations on a second support frame that is separate from the first support frame. This means that the conveyor device has a support frame of its own, and the metal detector likewise has a support frame of its own through which it is supported on firm ground. With separate support frames, the propagation of vibrations from the drive mechanism of the belt to the metal detector is minimized. The arrangement and use of two or more weighing cells further offers cost savings, as fewer weighing cells need to be installed, for example, with narrow conveyor belts or if the articles are always positioned in the middle of the conveyor belt, in other words, in any applications where the weighing cells are not subject to the problem of eccentric loading.
According to another preferred embodiment of the invention, the second support frame comprises a first partial support frame and a second partial support frame, wherein each partial support frame includes at least one mounting location for a weighing cell.
In a further embodiment of the invention, the metal detector is mounted on a first support frame, while the second support frame is designed to support the conveyor device on the housing of the metal detector. The combined unit of metal detector and conveyor device can thus be disconnected from the mounting locations of the metal detector on the support frame and replaced by another such unit. This facilitates the exchange of the combined weighing/metal-detection system in case the production line has to be changed over to a product of significantly different weight and size.
In a further embodiment, the first partial support frame and the second partial support frame are tied together by a transverse connection.
Another preferred embodiment of the invention is distinguished by the fact that several conveyor devices run parallel to each other through the metal detector. An arrangement of several conveyor devices running parallel to each other through the metal detector not only increases throughput but also reduces the purchase costs of a system. Since the production costs for a metal detector of smaller dimensions are not proportionally lower, there is a relative cost disadvantage for a weighing/metal-detection system for smaller articles. This disadvantage can be overcome by selecting a metal detector of larger dimensions with a plurality of conveyor devices running through the same metal detector.
Each conveyor device in such an arrangement may rest on at least two weighing cells, so that weight checks can be performed separately on each conveyor device. The metal detection may be closely synchronized with the weighing at the time when the weighing object on the conveyor device passes through the metal detector. The arrangement and use of two or more weighing cells further offers cost savings, as fewer weighing cells need to be installed, for example, with narrow conveyor belts or if the articles are always positioned in the middle of the conveyor belt, in other words, in any applications where the weighing cells are not subject to the problem of eccentric loading.
In an advantageous further embodiment of the invention, the conveyor device is supported by three weighing cells representing a three-point support. A statically determined support which rests on at least one support frame and is safe from tipping over increases the accuracy of the weight measurement, for example, by minimizing eccentric load errors.
In another preferred embodiment of the invention, the conveyor device is configured with four weighing cells arranged at the corners of the conveyor device. In the case of a wide conveyor belt, a weighing object positioned off the middle of the belt, transverse to the transport direction, can introduce a torque in the weighing cells and thereby compromise the measurement. With an arrangement of four weighing cells at the corners of the conveyor device, this problem can largely be eliminated.
According to a particularly advantageous development of the inventive concept, the metal detector is arranged midway between the ends of the conveyor device in the transport direction. In view of the metal-free zone (MFZ), the centered arrangement of the metal detector relative to the conveyor device allows the length of the latter to be minimized relative to the transport direction. Given that only one product at a time may be present on the conveyor for the weighing, this has the advantage that the cycle period is shortened, as the weighing objects travel through the weighing/metal-detection systems at closer intervals, so that the throughput is increased.
In a further advanced embodiment of the invention, a diverter system downstream of the weighing/metal-detection station is configured to remove those objects that are not in conformance with the given criteria in regard to weight tolerances or content level of metallic contaminants. In comparison to a solution with a separate checkweig her and metal-detector, the cost of one diverter system can thus be saved.
According to another embodiment of the invention, the conveyor device includes a conveyor body and a belt looping around the conveyor body for the transport of objects.
According to yet another preferred embodiment of the invention, the conveyor device is positioned so that it passes through the metal detector near the bottom corners of the opening. The reason for this is that the detection sensitivity is weakest at the geometrical center of the opening and strongest in the peripheral areas.
In addition to the novel features and advantages mentioned above, other benefits will be readily apparent from the following descriptions of the drawings and exemplary embodiments.